DNA adduction is a critical event for the carcinogenic process. Distribution of DNA adducts, mutational spectra, and DNA repair will be important factors in determining whether critical target genes are mutated. Methods for the analysis of heterocyclic arylamine (HAA) adducts in specific genes were developed using Southern hybridization techniques together with alkali or uvrABC excinuclease to excise at adduct sites. Studies examining the repair in transcriptionally active and inactive genes in Chinese hamster ovary cells are underway. Studies with a related compound, 4-nitroquinoline N-oxide showed that repair in transcriptionally active genes was the same as in nontranscribed genes. In contrast, studies underway with HAA adducts suggest that repair of these adducts is coupled to transcriptional activity of the gene. The spectrum of 2-amino- 3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline (MeIQx) adducts is being examined in two systems: in the DHFR gene of Chinese hamster ovary cells and the Sup-F shuttle vector system. In the DHFR gene of Chinese hamster ovary cells, PhIP produces predominantly G:T transversions in the nontranscribed strand, which is consistent with the notion that repair is higher in the transcribed than the nontranscribed strand. In parallel with studies in the DHFR gene, studies in the Sup-F shuttle vector system show that IQ also produces predominantly G:T transversions. Studies with PhIP are underway. Studies with the Sup-F shuttle vector system in repair deficient and proficient cells indicate that the spectra of mutations are influenced by the DNA repair capacity.